Enhanced corrosion of zirconium alloys may occur when the corroding surface is close to, or in contact with, certain other metallic components. The shape of the component is often reproduced in the shape of an area of enhanced corrosion, suggestive of a shadow cast by the component on the zirconium alloy surface. The term ‘shadow corrosion’ is therefore often used to describe the phenomenon. Observations of shadow corrosion on water reactor components have been noted for many years. In 1974, Johnson et al. (1974), reported enhanced corrosion in Zircaloy coupons located near, but not touching, small pieces of platinum in the advanced test reactor (ATR). Also, Trowse et al. (1977) reported enhanced fuel rod corrosion beneath steel spacers in steam generating heavy water reactors (SGHWRs).

Most commonly observed are the control blade shadows on BWR chan­nel surfaces adjacent to the control blade handles, such as shown in Fig. 4.53. In this case the stainless steel control blade handle is imaged as a black shadow on a light background, but the reverse is sometimes also observed. It was shown (Chen & Adamson, 1994) that the handle image is faithfully reproduced on the channel surface, but is larger than the actual handle, shown schematically in Fig. 4.54. Hot cell examination of a similar channel shows that oxide thickness within the shadow area can be much higher than outside the shadow (Adamson et al., 2000 ).

Channel

Control blade handle Control blade

CHANNEL *91695

4.53

Control blade shadow on a BWR channel. Upper diagram shows the relative arrangement of blade handle and shadow (Chen & Adamson, 1994). Copyright 1994 by the American Nuclear Society, La Grange Park, Illinois.

Zry-2 channel, 43 MWd/kgU

Oxide

20 ..m

Away from shadow area 20 .m, 300 ppm H2

(b)

Oxide

20 .m

In shadow area 12 .m, 150 ppm H2

4.55 Oxide thickness (a) away from and (b) within a control blade handle shadow (Adamson et al., 2000).

Figure 4.55 illustrates a 6x difference in oxide thickness within and with­out of the shadow. It is noteworthy, however, that in this case, hydrogen content in the shadow is 150 ppm, while outside the shadow it is 300 ppm. Since temperature gradients are small in a channel wall, redistribution of hydrogen by thermal-gradient driven diffusion is also small, indicating in this case a much reduced hydrogen pickup rate in the shadow. More recent data indicate variability in pickup fraction (HPUF), as (Mahmood et al., 2010) reported normal HPUF. It is also observed that control blade shadows preferentially form during the first cycle of operation and that the thickness tends to saturate with burnup.

Shadows on fuelled or non-fuelled rods have been observed under Zircaloy spacers with Inconel springs or under all-Inconel spacers (Fig. 4.56). Again,

BWR shadow corrosion 4.56 Shadow corrosion data of various BWR fuel vendors’ claddings. (Source: Figure modified according to Hoffmann and Manzel (1999); Potts (2000); Zwicky et al. (2000). Copyright 2000 by the American Nuclear Society, La Grange Park, Illinois.)

this illustrates the trend for saturation of the oxide thickness with fluence or burnup. In most cases, shadows have not caused fuel performance problems. The upper curve in Fig. 4.50, is an exception for a specific cladding condition, called Enhanced Spacer Shadow Corrosion (Zwicky et al, 2000).

A number of experiments have been conducted to elucidate the details of the shadow corrosion mechanism. Combined with the commercial reactor observations, these experiments reveal:

1 A variety of metals are observed to cause shadows on Zircaloy. These are:

(a) Stainless steel (many)

(b) Pt (Shimada et al, 2002; Johnson et al, 1974)

(c) Hf (Shimada et al, 2002)

(d) Inconel X750, X718 (many)

(e) Inconel 600 (Adamson et al, 2000)

(f) Welded regions on Zircaloy (Chen & Adamson, 1994; Shimada et al, 2002).

2 Nitronic 32 was observed not to cause shadows (Andersson et al, 2002).

3 Resistance to shadow formation depends upon inherent corrosion resis­tance (Andersson et al, 2002; Garzarolli et al, 2002; Shimada et al, 2002).

4 The distance between components is critical. Oxide thickness is a func­tion of distance. There is a maximum distance above which there is no effect (a few mm) but there is no minimum distance, including touching (Chen & Adamson, 1994; Lysell et al, 2001; Andersson et al, 2002).

5 In general, the shadow forms at low fluence or exposure and thickness of the shadow saturates with fluence (Fukuya et al, 1994; Hoffmann & Manzel, 1999; Andersson, 2000). In unusual cases, perhaps due to spe­cial microstructure and water chemistry, accelerated shadow corrosion begins at high fluence or burnup (Zwicky et al, 2000; Wikmark & Cox, 2001 ; Andersson et al., 2002 ).

6 Shadow formation requires a nuclear reactor environment and it has not been possible to reproduce it in laboratory autoclaves (Andersson, 2000; Garzarolli et al;, 2001). However, use of ultraviolet light in the labora­tory has been shown to increase electro-chemical potentials between common components, believed to be related to shadow formation (Kim et al., 2010 ).

7 No reports of shadow formation have been made for PWRs or high — hydrogen cases. BWR hydrogen water chemistry conditions, however, do allow shadows (Lefebvre & Lemaignan, 1997; Adamson et al, 2000).

8 Thick oxide shadows do not necessarily result in proportionally high hydrogen pickup and can result in unusually low hydrogen pickup (Adamson et al, 2000) or normal pickup (Mahmood et al, 2010).

9 Pre-oxidation autoclaving of Zircaloy does not prevent shadows, but applying a zirconia layer to Inconel does (Andersson et al, 2002).

10 Shadow corrosion has been observed when the two metals are not in contact physically and are nominally electrically insulated from each other. However, in making such observations it has been assumed that the radiation field has no effect on the conductivity of the insulating medium.

11 Shadow formation has been reported (Chatelain et al, 2000; Andersson et al. , 2002) in a reactor position outside and downstream of the MIT test reactor core where the neutron and gamma fluxes are reported to be near zero. On the other hand, no shadows were reported (Lysell et al., 2001) in a reactor position outside and upstream of the R2 test reactor core where the neutron flux and gamma power (flux) were also near zero. However, there does appear to be some uncertainty in the actual gamma intensities in the MIT experiment, so there may need to be a re-evaluation of the out-of-core results.

12 Shadow formation can be prevented if electrical connection between the two components is prevented (Lysell et al., 2005 ).

The early thoughts on the mechanism of shadow corrosion centred on it being a form of galvanic corrosion. As such, the mechanism required a path for electron transfer between the cathodic shadower (the material that causes the shadow) and the anodic component (the Zircaloy or zirconium alloy component on which the enhanced corrosion occurs) and a conduc­tive path through the water separating the two parts. But since the shadow phenomenon could not be reproduced in the laboratory, it was clear that some sort of radiation effect was also required. Problems with the galvanic hypothesis included lack of evidence, in some cases, of any electrical con­nection between the shadower and component (although for commercial reactor components an obscure path can always be suspected) and concern that the zirconium oxide, which is always present on component surfaces, was not conductive enough to allow the postulated conductive paths to operate.

Another hypothesis arose when Chen and Adamson (1994) noted that the range in water of beta particles from Mn-56 and Zr-97 (originating in the shadower material) could explain the shape and size of observed shadows if a beta-damage mode could be found. Lemaignan (1992) proposed that extra radiolysis caused by the imposed local beta flux could result in accelerated corrosion. However, additional calculations by Andersson et al. (2002) and Shimada et al. (2002) indicated that the extra beta flux from the shadower does not make a significant change to the overall beta flux in the reactor, so this hypothesis has been discounted. Also, as noted above, it has been shown that the alloy Nitronic 32 does not cause shadows, even though the flux of beta particles from activated Mn-56 from that alloy is much higher than for the known shadowers Inconel and stainless steel.

The latest view of the mechanism is that it is indeed a form of irradiation — assisted galvanic corrosion. Points which support this hypothesis include:

1 It is known that there is a corrosion potential difference between, for instance, stainless steel or Inconel and Zircaloy in non-hydrogenated water (BWR type) (Table 4.9). Also, this potential difference is enhanced in-reactor (Lysell et al., 2001) and by ultraviolet light outside the reactor (Kim et al., 2010 ).

2 The observed relationship between component separation distance and shadow oxide thickness is roughly as expected (Adamson et al., 2000 ; Lysell et al., 2001; Andersson et al., 2002).

3 A true stainless steel/Zircaloy galvanic couple in-reactor produced thick oxide and low hydrogen pickup in Zircaloy, similar to that observed for a control blade handle/Zircaloy shadow (Adamson et al, 2000; Lysell et al, 2005).

4 A radiation enhancement of electrical conductivity of oxides has been reported. Electrical conductivity of oxide films on Zircaloy markedly

Table 4.9 Corrosion potential differences (mV) between Zircaloy-4 and Inconel in BWR and PWR environments Environment Material coupling Inconel-Zry-4 ground Inconel-Zry-4 pickled PWR type -60 -75 BWR type -420 -640 Source: A. N.T International (2011) and Garzarolli et al. (2001b).

increased during electron irradiation (Howla et al., 1999), gamma irra­diation (Kang et al, 1994) and in-reactor (Shannon, 1962). Also, the con­ductivities of various ceramics, including Al2O3 were reported to increase dramatically under proton or x-ray irradiation (Hobbs et al, 1994). So perhaps a way to allow closing of the galvanic circuit is indicated. Note that Al2 O3 was used as insulation in the MIT experiment (Andersson et al, 2002) and ZrO2 as insulation in others (Chen & Adamson, 1994; Shimada et al;, 2002). Some distinction must be made between surface conduction and bulk conduction in thick ceramics, but little is known.

5 Shadows were not formed when the components were confirmed to be electrically insulated (Lysell et al, 2005).

Points which bring doubt to the irradiation-assisted galvanic mechanism hypothesis include:

6 It is not certain that an electrically-conducting path truly exists between the shadower and component.

7 Conventional galvanic reactions are inhibited when the cathode is small and the anode is large, as is often the case with observed shadows.

8 The MIT experiments (Andersson et al, 2002) appear to produce shad­ows in a region of very low radiation.

It is also quite possible that several different mechanisms contribute to shadow corrosion depending on specific conditions.

It is obvious that the mechanism of shadow corrosion is not precisely defined, although the number of observations which help to provide a work­ing hypothesis are many. It is clear that enhanced corrosion can and will occur in cases where Zircaloy and a variety of other metals or alloys are in close contact. Potential problems include loss of strength and integrity due to wall thinning and oxide spallation. In most cases, the enhanced corro­sion does not cause serious operational problems. The oxide thicknesses are moderate, hydrogen absorption is not increased by straightforward shadows and spalling usually does not occur. The severe problem of enhanced spacer shadow corrosion (ESSC)) can apparently be mollified by control of both Zircaloy microstructure and by reactor water chemistry. No harmful effects have been reported or been related to the use of Pt in noble water chem­istry treatments in BWRs, but further studies are underway. BWR channel bowing has been attributed to manifestations of shadow corrosion, and has produced major in-reactor problems to be discussed later in this chapter, but further study is required to understand the details (Mahmood et al, 2007; Blavius et al, 2008; Munch et al, 2008). There does not appear to be an immediate remedy for shadow corrosion — for instance an oxide prefilm on Zircaloy does not prevent shadows. Coatings on the shadower (such as Inconel springs) would appear to be promising, but may not be practicable.